Laine, J

Stenius, P

Carlsson, G

Ström, G

Abstract [en]

The effect of digestion conditions (amount of effective alkali, digestion time) on the surface compositions of unbleached softwood (Pinus sylvestris) kraft pulp has been investigated by ESCA analysis. The quantities monitored were the angular dependence of the total O/C ratio, the relative amounts of carbons in different states of oxidation and the adsorption of Al and Ca ions to the carboxyl groups in the surface. Analysis of the angular dependence of ESCA intensities shows that the concentration of alkyl carbon is high in a very thin surface layer. This enrichment becomes more marked as the lignin content (kappa number) decreases, but it is not affected by extraction of the fibres with dichloromethane. It is concluded that the observed distribution is due to re-precipitation of lignin. In pulp that has not been extracted, there is also strong enrichment of extractives in the surface. This amount increases with increasing effective alkali but is relatively independent of the time of digestion. ESCA analysis of the Al and Ca bound to the carboxyl groups shows that the amount of these depends on digestion time; the results are consistent with the notion that the reprecipitated lignin contains carboxyl groups.